1. Field of the Invention
This invention relates generally to printing-press equipment, and more particularly to a "suction foot" or "sheet sucker" used in small presses to separate the top piece of stock (paper, cardboard, etc.) from a pile and to forward that top piece into the press proper.
2. Prior Art
There are basically three known forms of suction foot for a printing press. The first is simply a cut-off hollow rod, attached at the top to a suction system tnat is part of the press (or is added later). Attached to the bottom tip of the rod there is usually a soft rubber or plastic hood that droops slightly onto the stock to enhance the suction effect between the tip of the rod and the stock.
Such a foot is moved by the press mechanism in such a way that it slightly lifts the top sheet of stock from the supply pile, or moves the top sheet forward after it has been lifted, or both. In the printing craft the lifting process is generally called "separation"; the forward motion, which moves the sheet into a position where it can be pulled along by rollers in the press, is generally called "forwarding".
Simple cut-off rods are problematical in that they must be positioned very carefully for each project (and for each sheet), to apply suction effectively without punching holes or indentations into the stock. This is particularly difficult since the height of the pile of stock changes continuously during operation of the press, and the servocontrolled stock elevator cannot perform perfectly.
Cut-off rods are also troublesome because they tend to compress the top of the pile slightly, pushing air out of the pile. Effective separation requires ready access of air under each of the top few sheets--to push the top sheet upwardly, while the suction system removes the downward pressure normally presented by air above the top sheet.
Nevertheless, such suction feet are used widely in small presses for their extreme simplicity and low cost--but most particularly because they do not take up much room. The simple rod-type foot does not extend horizontally toward the pullout rollers, various crossbar mounts, or side guides and side walls in the rather cramped quarters within a small press, and its suction hose extends vertically, up out of the way of these various other components.
Hence design efforts have been concentrated in providing adjustments for the rod-type suction foot, relative to the top of the supply pile, that are as accurate as such things can economically be. Such adjustments sometimes require considerable "fussing" by the press operator at the beginning of each project, and continual vigilance to be certain that the stock is not being damaged by drift of the adjustment, or by excessive dead zone in the elevator servosystem.
A second generation of suction foot is known as the "compensating foot" or, more affectionately, "compensating sucker". The compensating type has a vertical cylinder, with a hollow internal piston and rod that are within the cylinder and are spring-loaded downwardly relative to the cylinder. At the lower tip of the piston rod there is of course a floppy hood as before. At the top of the cylinder is a hollow support rod (similar to the cut-off rod foot) that is attached to the suction system of the press. The hole in the support rod communicates directly witn the space above the piston, and hence communicates with the hole through the piston and rod.
In operation the compensating sucker is lowered by the press mechanism toward the pile of sheets, if it is being used for separation. If the sucker is being used for forwarding only, a sheet is raised toward the compensating sucker. In either case, before the solidly supported cylinder hits the top sheet, the downwardly spring-loaded piston-rod tip engages the sheet, closing the end of the air passage. The suction now becomes effective to hold the sheet of stock up against the tip, but with much less fussiness of adjustment since the tip may engage the sheet anywhere within the piston stroke.
For the compensating sucker to work well in a separating mode, however, it is also necessary for the suction system to draw the piston upward within the cylinder. In other words, once the sheet of stock has closed off the bottom of the piston-rod tip, the dead air space within the piston and piston rod act almost as if they were part of the piston. The situation is theoretically as if the piston were solid, and the suction applied above the piston should retract the tip. In theory the sheet is thus raised from the top of the pile.
In practice these devices are unreliable, because the suction force for retraction is opposed both by the spring and by gravity. The pressure differential available from a typical suction system often varies in operation, and especially with age; and the weight of a sheet of stock varies substantially. Small pieces of dirt and the like can increase stiction in the system, making matters worse.
The overall result is unreliable retraction. In addition the compensating sucker if used for both separation and forwarding often drags on horizontal return, damaging or marking the next sheet. Furthermore, the downwardly spring-loaded foot sometimes presses too hard on the pile, before the suction takes effect to retract the foot. In such cases air is squeezed out of the top of the pile, impairing separation as with a cut-off rod.
A third type of suction foot, sometimes called a "sheet seeker", has been previously found only on large presses, and has been used almost exclusively for forwarding. In addition, the sheet-seeker type has been used only to support a sheet from generally the center of the sheet (relative to the direction of feed), rather than from the leading edge. Finally, sheet seekers have been used only in "stream feeding" systems--systems in which successive sheets are overlapped for travel into the rollers of the press, rather than being strictly consecutive as in a pure "sheet feeding" system.
Except for these severe limitations in application, the sheet-seeker foot operates very well. It too has a cylinder, spring-loaded hollow piston and piston rod, and suction-system attachment. The spring-loading, however, is upward rather than downward, and the suction-system connection point is not through the top support rod but rather by means of an external port partway down the side wall of the cylinder.
Thus the force relationships are reversed: it is the suction system (aided by gravity) that extends the suction tip downward toward the sheet, by drawing downwardly on the underside of the piston; and it is the spring that retracts the tip after the sheet is engaged. In this system there is a very fine pilot-pressure hole drilled radially through the piston-rod wallp--so that, when a sheet closes the bottom of the hollow piston rod, air is removed from the central hollow, equalizing the forces on the top and bottom of the piston. In some cases the same effect may be obtained by relying on leakage around the piston. The suction effect on the piston is thereby neutralized, and the spring drives the piston upward to retract the tip with the suspended sheet.
This operation has been found to be excellent, and sheet-seeking feet consequently are becoming generally standard on large presses--but, as previously mentioned only for center-of-sheet support and stream feed. They have not been used, even in large presses, for leading-edge support or sheet feed. Moreover, they have been only negligibly used for separation--as, for example, in certain Miehle units where there is a small amount of vertical motion and a large amount of horizontal motion.
The reason for nonuse of sheet seekers in small presses is fairly clear: prior sheet-seeker feet have been large, and their suction lines protruding from the sides of the cylinders have made them entirely impractical for small presses. By "small" I refer to presses capable of printing sheets no larger than, roughly fourteen by eighteen inches (roughly thirty-five by forty-five centimeters).
Many small presses such as the ATF Chief are particularly designed for (and particularly effective with) leading-edge sheet-feed forwarding. There would be inadequate room for the cylinder of a conventional sheet sucker, not to mention a forward-extending suction hose, anywhere near the leading edge of a sheet that is being individually (i. e., sheet-feed) forwarded. It will be understood that in stream feeding the leading edge of each sheet is carried into the press proper by its central or rearward portions. Consequently the suction foot need not move as far forward toward the pullout rollers and related hardware, and there is a lesser problem of clearances.
Even if the bulkiness of the cylinder could be overcome, prior sheet-seekers would still be extremely problematical in most or all small presses because of the suction line placement. For definiteness in the following discussion of clearances, the ATF Chief will be used as a point of reference. It will be understood, however, that the clearance problems as described are merely exemplary of analogous problems to be found in small presses generally.
If the suction line were extended forwardly from the cylinder wall, it would drag on the lower pullout roller or in some other way--as already suggested--aggravate the leading-edge clearance problem. If the suction lines for a pair of sheet-seeker feet were extended laterally "outward" (toward the side walls of the press) there would be a problem of clearing the side guides when small pieces of stock were being printed. There would be a problem of clearing the side walls, and related hardware there, when fairly large pieces of stock (approaching the lateral size capacity of the machine) were being printed.
On the other hand, if the lines for a pair of sheet seekers were extended laterally "inward" (toward each other), then there would be mutual interference, and interference with the upper pullout roller in many cases, for relatively small pieces of stock (such as envelopes and small announcements). If the lines were extended rearwardly--paralleling the direction of feed--there would be interference with the pile-height sensor, and/or with the transverse shaft that supports the side and rear guides.
The upshot of this discussion will be a realization that in a small press the suction hoses for even a slimmed-down version of the prior-art sheet seekers would have to be custom-rerouted for each job. For many jobs different hose lengths would have to be custom-installed. This kind of added chore would be an extreme aggravation to the pressman, and for competitive short-run commercial work would be totally uneconomical.
Furthermore, some small presses such as the AB Dick or Ryobi have suction feet that are a screw-in type. These are cut-off rods that thread into a hollow transverse bar, which supplies the suction at the top of the rods. Conventional sheet-seeker feet, with their lateral suction nipples, could not be mounted without custom modifications to the press.
It is not entirely clear to me why sheet-seeker feet have not been used in large presses for leading-edge support or sheet feed, or more extensively than they for separation, since many of the problems mentioned above would be alleviated by the more-ample clearances available in larger presses. I believe, however, that the reason is a combination of (1) the very awkward bulk of the conventional sheet-seeker configuration--particularly as to the hose-routing problem--and (2) the compound motion that is imparted to a suction foot when it is used for separation and forwarding in combination.
Restriction of the benefits of the sheet-seeker format to forwarding is very unfortunate. It is true that they are beneficial in forwarding because they are much less likely to drag on return, but sheet seekers are also particularly useful in separation. If they are not used, other types of suction foot must be used in tandem with the forwarding sheet seekers, to raise the sheets into engagement with the sheet seekers.
Such conventional cut-off rods or compensating suckers, in tandem with the sheet seekers, have all the previously enumerated disadvantages of damaging the stock or marring the finished work. These drawbacks are here compounded, however, by the big-business, high-pressure psychological environment that goes with operating the larger, more expensive equipment with its higher overhead.
Restriction of the benefits of the sheet-seeker format to center-of-sheet suspension is also undesirable, in comparison with leading-edge suspension, for the following reasons. In some multipass (e. g., multicolor) printing jobs, when the foot picks up the sheet in the printed area, incompletely dried ink tends to collect on the suction foot. This ink is then transferred to other sheets, marring the finished product. In other cases, spray powder that is used to hasten drying can collect on the suction foot, and be carried into the press--and thus into contact with the printing surfaces (e. g., the "blanket" in an offset lithographic press). Of course the powder interferes with the printing process, producing a ring pattern of the spray powder on the printed piece--but it can also interfere with the inking process, and on long runs the result can be a full-blown mess.
Finally, restriction of the sheet-seeker foot to stream feed is regrettable since accurate, positive feed can be obtained more economically in more modest presses that are designed for sheet feeding. Even the large presses that are sheet fed--such as one Heidelberg unit--do not use sheet-seeker feet.
Prior-art sheet seekers offer excellent performance relative to other suction-foot types; however, all the characteristics that make them incompatible with small presses, with separation, with leading-edge support, and with sheet feed, are serious disadvantages.